Externally splined fastener

ABSTRACT

A fastener ( 2 ) for securing a workpiece comprising a plurality of workpiece members, said fastener comprising a shank ( 4 ) and a radially enlarged head ( 6 ) wherein the fastener further comprises axial voids ( 40 ) and splines ( 18 ), such that when the fastener is installed into a workpiece by a mandrel, breakstem or pin, the shank of the fastener is expanded such that the crests of the splines are caused to mechanically engage with the internal walls of the fastener apertures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a national phase filing of International ApplicationNo. PCT/GB2011/051107, filed on Jun. 14, 2011, which claims priority toGreat Britain Application No. 1012298.4, filed on Jul. 22, 2010, each ofwhich is incorporated herein by reference.

BACKGROUND AND SUMMARY

This invention relates to a speed fastener for securing aperturedworkpiece members together.

Speed Fastening™ is a well known method of securing workpiece memberstogether, whereby a fastener having a hollow core is placed in alignedapertures in the workpiece members, and a headed mandrel is pulledthrough the core of the fastener to cause radial expansion of thefastener shank, and additionally a small degree of foreshortening of thefastener. The radial expansion of the fastener shank ideally achievesmechanical engagement between the fastener shank and the walls of theapertures in the workpiece members. The axial foreshortening effect canhelp pull the rearmost sheet up towards the fastener head to close gapsbetween the workpiece members.

Prior art fasteners, such as those available under the trade marks Briv(UK patent number GB1323873A) and Chobert, have cylindrical shankportions which in a central region have a uniform wall thickness, i.e. aconstant cross-sectional area. The tail end of the fastener shank has anincreased wall thickness; in the case of the Briv fastener this is anenlarged outer diameter and in the case of the Chobert fastener, areduced bore diameter of tapered form.

These prior art fasteners are installed by pulling a mandrel of a givendiameter through the fastener bore, thereby causing a radially enlargedhead of the mandrel to expand the central region of the shank into theapertures in the workpiece members. Accordingly, the degree of radialexpansion is governed by the diameter of the mandrel head for a nominalfastener diameter.

Prior art fasteners have a low degree of tolerance in workpiece aperturediameter, i.e. only a limited range of workpiece aperture diameters canbe accommodated with a nominal fastener size and a single head diametermandrel. Accordingly, there is a narrow range between the two acceptableextremes of workpiece aperture diameter when using a nominal fastenerand a specific head diameter mandrel.

In an optimised installation of a prior art fastener, i.e. wherein theworkpiece aperture diameters are within the acceptable range for a givenfastener diameter and mandrel head diameter, the expansion of thefastener shank by the mandrel head causes mechanical engagement betweenthe fastener shank and the walls of the workpiece apertures, without‘overpacking’ (as explained below). The constraint provided by the wallof the aperture of the rear workpiece member (i.e. the workpiece memberwhich is furthest away from the fastener head) generates radial forcesand hence frictional resistance to axial movement of the rear workpiecemember relative to the fastener rivet shank.

However, if a workpiece aperture diameter is smaller than the acceptablerange for a prior art fastener, an unduly high placing force will berequired to install the fastener because it will be overpacked in theaperture. This can cause wear of the mandrel or cause other undesirableeffects such as debris generation from the fastener bore, or fastenerhead malformation, such as by extrusion of fastener bore material upinto the head region.

It is common for the aperture diameter of the top workpiece member (i.e.the workpiece member which is closest to the fastener head) to beconfigured such that it is slightly larger than the aperture in the rearworkpiece member, in order to accommodate aperture pitch errors. If theaperture diameter of the top workpiece member is above the acceptablerange for a prior art fastener, the expansion of the shank of thefastener does not result in any mechanical engagement between thefastener shank and the wall of the oversized workpiece aperture,resulting in low resistance of the joint to movement under shear loads.

To compensate for oversize workpiece apertures, an oversized mandrel,i.e. a mandrel having an oversized head, can be used to install afastener. However, there is a risk that an oversized mandrel canincorrectly be used for a undersized workpiece aperture, thereby leadingto the problems caused by a high placing force as discussed above.

The problem of a workpiece aperture diameter being above the acceptablerange for a specific fastener diameter and mandrel head diameter isillustrated in FIG. 1, which illustrates a prior art fastener 2installed into a workpiece 30 at the lowest end of a grip range. Theworkpiece comprises a top workpiece member 32 having an oversizedaperture 36, and a rear workpiece member 34 having an acceptableaperture 38. Due to the top workpiece member 32 having an oversizedaperture 36, the installation has not resulted in any mechanicalengagement between the fastener shank 4 and the wall of the aperture 36.

A correctly installed prior art fastener exhibits a enlarged tail endformation due to the greater wall thickness in that region. In amaximum-grip application, this enlarged tail end formation is adjacentto the outer surface of the rear workpiece member and so serves toprovide support and resistance to movement of the joint if subject totensile loads. However in a mid-grip or a minimum-grip condition (asshown in FIG. 1) the enlarged tail formation 44 is remote from the rearworkpiece member 34 and it is therefore unable to help resist initialseparation of the workpiece members 32, 34 under tensile loads; theresistance must result almost entirely from the frictional forcesresulting from the radial pressure exerted by the expanded centralregion of the fastener shank 4 having a uniform wall thickness.Consequently the resistance to separation is lower in mid orminimum-grip conditions.

Other speed fasteners which are currently available, such as thoseavailable under the trade marks Avtronic™ and Rivscrew™, feature anon-uniform wall thickness in the centre of the fastener shank. Thesefasteners comprise voids on the exterior of the fastener shank in theform of annular grooves and a helical screw thread form respectively.These fasteners are intended to expand and embed into a workpiecewherein the workpiece members are comprised of a material which isrelatively soft in comparison to the material of the fastener. Theannular or helical voids of these fasteners lead to variable expansioninto the rear workpiece member, with high or low degrees of mechanicalengagement between the fastener shank and the wall of the workpieceapertures, and an expanded tail form of inconsistent size ordistribution around the hole periphery.

The present invention is aimed at providing a fastener which will ensurecomplete and consistent mechanical engagement between the fastener shankand the wall of both the workpiece apertures, i.e. achieve completemechanical engagement for a fastening which is more robust than isachievable with prior art fasteners. The present invention is also aimedat providing a fastener which allows the specification of a singlefastener length and one mandrel size to install into a wider grip rangeand which can tolerate a far greater variation in workpiece aperturediameter than prior art fasteners, whilst avoiding the problems of highplacing force, mandrel wear, debris generation and fastener headmalformation with a single mandrel head size or diameter.

Furthermore the present invention is aimed at providing a fastener whichis suitable for use in securing workpiece members which may be of hardermaterial than the fastener, and wherein the grip range of the fasteneris dependent only upon the length of the fastener i.e. a longer fastenerhas a greater grip range, allowing a fastener having a nominal length tobe used in a wider range of workpiece thicknesses.

Accordingly the present invention comprises, in a first aspect, afastener, for securing a workpiece comprising a plurality of workpiecemembers, said fastener comprising a shank with a head end and a tail endremote from the head end, and a radially enlarged head at the head endof the shank, and a bore extending throughout the fastener, wherein anexternal wall of the shank comprises a voided section comprising aplurality of voids separated by a plurality of splines each of whichculminates in a crest, the voids and splines being elongated in an axialdirection with respect to a longitudinal axis of the fastener bore;wherein the voids are of an approximate trapezoidal cross-section, andwherein the voids have a cross-sectional area which is constant alongthe length of the voided section of the fastener shank.

The present invention comprises, in a further aspect, a method ofinstallation of a fastener, thereby to secure a workpiece comprising afirst apertured workpiece member and a second apertured workpiecemember, the method comprising the steps of; a) placing a mandrel havinga radially enlarged head through the fastener bore such that theradially enlarged head of the mandrel is adjacent the tail end of thefastener shank; b) inserting the fastener and mandrel into the aperturesin the first and second workpiece members such that the fastener headcontacts the first workpiece member; c) supporting the fastener at thehead end whilst drawing the mandrel entirely through the fastener bore,thereby expanding the fastener shank into the apertures of the workpiecemembers, and causing the crests of the splines to be deformed againstthe apertures, and causing the tail end of the fastener shank toradially enlarge adjacent to the second workpiece member.

The installation of the fastener may be undertaken by a tool comprisinga conical concave end face which bears on an upper surface of thefastener head. This causes the fastener head to be pushed towards thetop workpiece member and the head to flatten slightly against the topworkpiece member, and thereby ensuring clamping of the fastening. Theflattening of the head periphery against the top workpiece member actssuch that the outer diameter of the underhead recess decreases, and thebearing area of the fastener head against the workpiece is increased.Furthermore the bearing area of the tool end face on the upper surfaceof the fastener head is greater than with prior art speed fasteners suchthat undesirable indentations or damage to the surface of the fastenerhead are minimised during installation of the fastener.

The present invention comprises, in further aspects, a method ofinstallation of a fastener, thereby to secure a workpiece comprising afirst apertured workpiece member and a second apertured workpiecemember, the method comprising the steps of; a) inserting a stemcomprising a radially enlarged head, a breakneck point, and a pluggingportion, into the fastener through the fastener bore such that theradially enlarged head of the stem is adjacent the tail end of thefastener shank; b) inserting the fastener and the stem into theapertures in the first and second workpiece members such that thefastener head contacts the first workpiece member; supporting thefastener at the head end whilst pulling the stem with respect to thefastener head, thereby causing the plugging portion to radially expandthe fastener shank into the apertures of the workpiece members anddeform the crests of the splines against the apertures, and also therebycausing the radially enlarged head of the stem to contract, and thencompress, the tail end of the fastener shank, thereby causing the tailend of the fastener shank to radially enlarge adjacent to the secondworkpiece member; wherein the stem is pulled with respect to thefastener head through the fastener bore until the stem fractures at thebreakneck point or, thereby to secure a workpiece comprising a firstapertured workpiece member and a second apertured workpiece member, themethod comprising the steps of; a) inserting the fastener into theapertures in the first and second workpiece members such that thefastener head contacts the first workpiece member; b) inserting abreakstem comprising a plugging portion with a hollow core, an elongateshank, and a breakneck point between the plugging portion and theelongate shank, into the bore of the fastener, such that the pluggingportion of the breakstem contacts the tail end of the fastener shank; c)pulling the elongate shank of the breakstem pulled relative to thefastener, thereby causing the breakstem plugging portion to enter thefastener shank, causing the fastener shank to expand into the workpieceapertures, and causing the crests of the splines to be deformed againstthe apertures, and simultaneously causing the plugging portion tocollapse inwardly; wherein the elongate shank is pulled relative to thefastener until the breakstem fails at the breakneck point.

A fastener installed by the method of installing a fastener, thereby tosecure a workpiece comprising a first apertured workpiece member and asecond apertured workpiece member, the method comprising the steps of;a) inserting the fastener into the apertures in the first and secondworkpiece members such that the fastener head contacts the firstworkpiece member; b) inserting a breakstem comprising a plugging portionwith a hollow core, an elongate shank, and a breakneck point between theplugging portion and the elongate shank, into the bore of the fastener,such that the plugging portion of the breakstem contacts the tail end ofthe fastener shank; c) pulling the elongate shank of the breakstempulled relative to the fastener, thereby causing the breakstem pluggingportion to enter the fastener shank, causing the fastener shank toexpand into the workpiece apertures, and causing the crests of thesplines to be deformed against the apertures, and simultaneously causingthe plugging portion to collapse inwardly; wherein the elongate shank ispulled relative to the fastener until the breakstem fails at thebreakneck point may further comprise a parallel portion between thebreakneck point and the plugging portion, which, during installation,forms a locking skirt thereby providing a mechanical lock of theinstalled stem in the fastener shank.

The present invention comprises, in a further aspect, a method ofinstallation of a fastener, thereby to secure a workpiece comprising afirst apertured workpiece member and a second apertured workpiecemember, the method comprising the steps of; a) inserting the fastenerinto the apertures in the first and second workpiece members such thatthe fastener head contacts the first workpiece member; b) driving asolid pin, having a maximum diameter larger than that of the fastenerbore, into the bore of the fastener via the head thereby to radiallyenlarge the fastener shank into mechanical engagement with the workpieceapertures, and causing the crests of the splines to be deformed againstthe apertures. This method may further comprise a subsequent step ofbearing on the fastener head by a support sleeve, and removing the pinfrom the fastener bore.

The axial voids of the present invention allow a variable degree ofmechanical engagement between the fastener shank and the wall of theworkpiece apertures in different sizes of workpiece member aperture,such as an oversized aperture in the top workpiece member, therebyproviding mechanical resistance to shear movement, without leading to‘overpacking’, thereby avoiding potentially excessive placing loads,mandrel wear, debris generation and fastener head malformation.

Because the voids are positioned axially with respect to the fastenershank, they have a cross-sectional area which is constant along thelength of the fastener shank. This ensures that the degree of radialexpansion and the mandrel load are consistent, independent of gripthickness. The present invention can therefore be used in a greater griprange than that offered by equivalent-sized prior art fasteners.Furthermore the grip range which can be accommodated by a specificfastener is dependent only upon the length of the fastener, i.e. alonger fastener can accommodate a greater grip range.

Furthermore, the present invention provides that a single mandrel (i.e.a mandrel having a set head size) can be used to install a fastener intoa workpieces having a variety of aperture sizes.

The present invention also provides greater expansion of the tail end ofthe fastener shank adjacent to the rear workpiece member, throughout thegrip range, thereby providing increased resistance to separation of theworkpiece members under tensile loads.

The present invention is also suitable for use in securing workpiecemembers which may be of harder material than the fastener.

The axial voids are preferably equidistant from one another.

The axial voids preferably extend from immediately below the fastenerhead.

The fastener head may include a countersink in its top surface, thecountersink having an average diameter which is greater than thediameter of the bore of the fastener. The countersink could comprise aflat or a curved annular tapered wall, and a neck portion could beprovided between the countersink and the fastener bore. The countersinkand the neck portion both accommodate rivet body material which is drawnaxially through and into the rivet head by the passage of the enlargedmandrel head through the rivet bore. Furthermore, the countersinkcontrols the broach load on installation of the fastener, and avoidsexcess fastener material pulling out from the top of the installedfastener head.

The fastener may include a tapered point at the tail end of the fastenershank. An advantage of the tapered point feature is that it allowsfasteners to be nested together “head-to-tail” in a stack within a paperpod, etc, when a countersink is provided in the fastener head. Thisensures coaxial alignment of adjacent rivet bores which in turn makes iteasier to load the fastener stack as one onto a mandrel shank.Furthermore the tapered point also assists in locating the fastener inthe workpiece aperture, and reduces the height of the stack of fastenersprior to installation, thus permitting more rivets to fit a placing toolof a given length, and provides increased rigidity to the stack on therelatively flexible mandrel.

The fastener may also include a solid tail ring provided at the tail endof the fastener shank, between the voided section and the tapered point.The tail ring, despite its remote position from the rear workpiecemember, nonetheless contributes to the ultimate tensile strength of thefastener via its reinforced wall section.

The fastener may also comprise an end section at the tail end of thefastener beyond the tapered point, the end section having a straightannular wall which is parallel to a longitudinal axis of the fastener.The end section is further facilitates the storing of multiple fasteners“head-to-tail” in a stack prior to installation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly and with reference to the figures in which;

FIG. 1 is a side view of a prior art fastener installed into aworkpiece;

FIG. 2 is a side view of a fastener in accordance with the presentinvention prior to installation into a workpiece;

FIGS. 3a and 3b are an isometric views of the fastener of FIG. 2;

FIG. 4 is a cross-sectional view of the fastener of FIG. 2 along theline IV-IV;

FIG. 5 is a cross-sectional view of the fastener of FIG. 4 along theline V-V;

FIG. 6 is a cross-sectional view of the fastener of FIG. 2 along theline VI-VI;

FIG. 7 is a side view of a plurality of fasteners of FIG. 2 arranged ina stack;

FIG. 8 is cross-sectional view of the stack of fasteners of FIG. 7 alongthe line VIII-VIII;

FIG. 9a is an axial cross-sectional view of the fastener of FIG. 2installed into a workpiece wherein the aperture diameter of the topworkpiece member is oversized;

FIG. 9b is an axial cross-sectional view of the fastener of FIG. 2installed into a workpiece having a minimum grip;

FIG. 10a is a cross-sectional view of the installed fastener of FIG. 9aalong the line Xa-Xa;

FIG. 10b is a cross-sectional view of the installed fastener of FIG. 9balong the line Xb-Xb;

FIG. 11 is a cross-sectional view of the installed fastener of FIG. 9aalong the line XI-XI;

FIG. 12 is a cross-sectional view of the installed fastener of FIG. 9aalong the line XII-XII;

FIGS. 13 and 14 illustrate an alternative method of installation of thefastener of FIG. 2;

FIGS. 15 and 16 illustrate a further alternative method of installationof the fastener of FIG. 2;

FIG. 17 illustrates a further alternative method of installation of thefastener of FIG. 2;

FIG. 18 illustrates a alternative embodiment of fastener in accordancewith the present invention;

FIG. 19 is a cross-sectional view of the fastener of FIG. 18 along theline XIX-XIX;

FIG. 20 is a side view of a further alternative embodiment of fastenerin accordance with the present invention;

FIG. 21 is a cross-sectional view of the fastener of FIG. 20 along theline XXI-XXI;

FIG. 22 is an isometric view of the fastener of FIG. 20;

FIG. 23 is a side view of a plurality of fasteners of FIG. 20 arrangedin a stack;

FIG. 24 is cross-sectional view of the stack of fasteners of FIG. 23along the line XXIV-XXIV; and

FIG. 25 is cross-sectional view of a fastener, workpiece, tool nose(remaining tool is omitted for clarity) and mandrel in accordance withthe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 2 to 8, a fastener 2 according to the presentinvention comprises a shank 4, a radially enlarged head 6 with a domedupper surface 7 at a head end 8 of the shank 4, and a tapered point 10at a tail end 12 of the shank 4. An annular underhead recess 42 isprovided under the head 6. A bore 14, having a diameter B, is providedthroughout the fastener 2, i.e. throughout the shank 4, head 6 andtapered point 10. A voided section 16 is provided on the external wall20 of the shank 4. The voided section 16 comprises a plurality ofsubstantial axial voids 40, which extend radially inwardly from themajor diameter ‘D’ of the shank exterior, and which separate a pluralityof substantial axial splines 18, which extend radially outwardly fromthe minor diameter ‘d’ of the shank exterior. The splines 18 areparallel and are spaced equidistant from one another around the externalwall 20 of the shank 4, and extend axially, i.e. with respect to alongitudinal axis of the fastener bore 14, from under the head 6 towardsthe tail end 12 of the shank 4. A tail ring 22 is provided at the tailend 12 of the shank 4, between the voided section 16 and the taperedpoint 10. The tail ring 22 comprises a solid wall, i.e. the voidedsection does not extend into the tail ring 22. A tapered transitionsection is provided between the voided section 16 and the tail ring 22,whereby at the end of each void 40 further from the head 6, the shankgradually increases in diameter to the major diameter ‘D’ of the shankexterior.

The splines 18 are of an approximate trapezoidal cross-section, andculminate in flattened crests 24, wherein a maximum shank diameter ‘D’is defined between the centres of opposing crests 24, and a minimumshank diameter ‘d’ is defined between the centres of opposing voids 40.The depth of the voids 40 (i.e. the height H of the spines 18 as shownin FIG. 4) is greater than would be present, for example, in knurling.The ratio of spline height H to maximum shank diameter D could be, forexample, approximately between 1:6 and 1:14.

A countersink 26 comprising a flat annular tapered wall is provided inthe top surface 5 of the head 6. The countersink 26 comprises a flat,annular tapered wall, such that the countersink has a maximum diameter Xfurthest from the fastener bore, and a minimum diameter x closest to thefastener bore. The average diameter of the countersink is thereforegreater than the diameter B of the fastener bore. A tapered neck portion28 is provided between the countersink 26 and the fastener bore 14,wherein the maximum diameter Y of the neck portion 28 is equal to theminimum diameter x of the countersink, and the minimum diameter y of theneck portion 28 is equal to the diameter B of the fastener bore.

Multiple fasteners are provided “head-to-tail” in a stack 50 asillustrated in FIGS. 7 and 8, ready for loading onto the mandrel or forsequential installation once on a mandrel within the placing tool. Themaximum diameter X of the countersink, and the angle of taper of thecountersink 26, are such that the tapered point 10 of an adjacentfastener can be accommodated at least partially in the countersink 26,allowing the fasteners to be stacked.

Installation of the fastener 2 to secure a workpiece 30 comprising afirst, top workpiece member 32 and a second, rear workpiece member 34(i.e. the workpiece member which is furthest away from the fastenerhead) is effected by inserting a mandrel 104 (FIG. 25) having a radiallyenlarged head 106 of maximum diameter M (where M is greater than thediameter B of the fastener bore 14), through the bore 14 of the fastener2, such that the radially enlarged head 106 of the mandrel is at thetail end 12 of the fastener shank 4. The fastener 2 and mandrel 100 areinserted into apertures 36, 38 provided in the workpiece members 32, 34.A tool having a tool nose 100 (the nose of the tool only is shown inFIG. 25) having a conical concave end face 102 is used to bear on thedomed upper surface 7 of the fastener head 6. The tool nose 100 is usedto pull the mandrel 104 through the fastener bore 14, thereby causingthe fastener shank 4 to expand and cause mechanical engagement betweenthe crests 24 of the splines 18 and the walls of the workpiece apertures36, 38 as the crests 24 are crushed against the walls of the apertures36, 38. The countersink 26 and the neck portion 28 both accommodatefastener body material which is drawn axially through and into thefastener head 6 by the passage of the enlarged mandrel head 106 throughthe fastener bore 14. Furthermore, the countersink controls the broachload on installation of the fastener 2, and avoids excess fastenermaterial pulling out from the top of the installed fastener head 6.

FIG. 9a illustrates the fastener 2 fully installed into a workpiece 30having a maximum grip, and FIG. 9b illustrated the fastener 2 fullyinstalled into a workpiece 30 having a minimum grip.

The force applied by the conical concave end face 102 of the tool nose100 to the domed upper surface 7 of the fastener head 6 causes the head6 to be pushed towards the top workpiece member 32 and the head toflatten slightly against the top workpiece member 32, and therebyensuring clamping of the fastening. The flattening of the head periphery22 against the top workpiece member 32 acts such that the outer diameterof the underhead recess 42 decreases, and the bearing area of thefastener head 6 against the workpiece 30 is increased. Furthermore thebearing area of the tool end face 102 on the upper surface 7 of thefastener head 6 is greater than with prior art speed fasteners such thatundesirable indentations or damage to the surface of the fastener head 6are minimised during installation of the fastener.

The extent to which the crests 24 are crushed against the a wall of aworkpiece apertures 36, 38 during installation is determined by thediameter of the aperture. An oversize workpiece aperture will result inthe crests 24 being only slightly crushed against the wall of theworkpiece aperture 36, 38. This is illustrated in FIG. 11, whichillustrates marginal crushing of the crests 24 against the wall of theaperture 36 of the top workpiece member 32. As illustrated in FIG. 12,an aperture having a nominal diameter (i.e. which is smaller than theoversized aperture of FIG. 11), will result in a greater degree ofcrushing of the crests 24 against the wall of the workpiece aperture 38than occurs with the oversized aperture of FIG. 11.

The displaced material of the crushed crests 24 is accommodated by theaxial voids 40. For a minimum diameter workpiece aperture, the axialvoids 40 will be completely filled by displaced crest material.

The tail ring 22 helps support the ends of the splines 18 and preventsthem from folding “inwards” under tensile joint loads. After the initialresistance of the crest 24 of the spline 18 is overcome in very hightensile loading, the crest 24 are “peeled off” as the rear sheet slidesup to the expanded tail ring. At the tail ring 22, the resistance tofurther movement is higher as this portion is a solid ring, thereforeensuring the ultimate tensile strength of the installed fastener.

It is possible that fastener 2 and/or that the aperture 38 of the rearworkpiece member 34 are not concentric with the aperture 36 of the topworkpiece member 32. In this situation, the crests 24 of the splines 18will be crushed by a variable amount against the wall of the aperture36, thereby still ensuring mechanical engagement of all crests 24against the aperture wall, despite the eccentricity of the fastener 2and/or the rear workpiece member aperture 38 relative to and topworkpiece member aperture 36.

FIGS. 9a to 10b also illustrate an enlarged tail formation 44 whichresults from the installation of the fastener 2. Beyond the rearworkpiece member 34, the splines 18 are free to expand fully (see FIGS.10a and 10b ), thus creating a ‘footprint’ or enlarged tail formation 44which is adjacent to the rear workpiece member 34 and which is incontact with the rear workpiece member 34 (see FIG. 10a ). A pluralityof trapezoidal footprint contact areas 46 are formed. Because theenlarged tail formation 44 is formed adjacent to and in contact with therear workpiece member 34, it therefore provides a greatly increasedmechanical resistance to initial axial movement due to tensile loadingon the fastening than is provided by prior art fasteners. The enlargedtail formation 44 is formed adjacent to the rear workpiece member 34across a given grip range, as can be seen in FIGS. 9a and 9 b.

In the above embodiment, the axial voids are provided by a plurality ofsplines of trapezoidal cross-section. However, in an alternativeembodiment, the axial voids may be of a different cross-section.

In the embodiment described above, the fastener is installed by pullinga radially enlarged mandrel head completely through the fastener bore14. However, alternative methods of expanding the fastener shank 4thereby to install the fastener 2 are illustrated in FIGS. 13 to 17. Themethod illustrated in FIGS. 13 and 14 uses a solid breakstem 70, whichcomprises a breakneck point 72, a radially enlarged head 74, andplugging portion 76 between the breakneck point 72 and the head 74,wherein the plugging portion 76 is of an equivalent diameter to themaximum diameter M of the mandrel head 106 of the used in the firstembodiment. The breakstem 70 and fastener 2 are inserted into theapertures 36, 38 in the workpiece members 32, 34. The breakstem 70 issubsequently pulled through the fastener 2 causing the plugging portion76 to radially expand the shank 4 of the fastener, until the radiallyenlarged head 74 of the breakstem 70 contacts with the tail end 12 ofthe fastener shank 4, which causes the load being applied to the stem 70to increase sufficiently to cause the stem 70 to fail at the breakneckpoint 72 FIG. 14 illustrates the fastener 2 fully installed into theworkpiece 30. The plugging portion 76 remains within the shank 4 of thefastener 2 and contributes additionally to the ultimate shear strengthof the installed fastener. Supplementary “barbs”, knurls or similar (notshown) could be provided on the plugging portion outer surface toprovided enhanced resistance against recoil of the stem plugging portion76 on failure of the breakneck point 72 and against removal of theplugging portion 76 from the fastener 2 after installation.

The alternative method illustrated in FIGS. 15 and 16 uses a breakstem80 such as that available under the trade mark Monobolt (UK patentnumber GB1538872). The breakstem comprises a plugging portion 82 with ahollow core 84, a parallel portion 85 adjacent the plugging portion 82,an elongate shank 88, and a breakneck point 86 between the parallelportion 85 and the elongate shank 88. Installation of the fastener intoa workpiece 30 involves inserting the fastener into the apertures 36, 38in the workpiece members 32, 34, the breakstem is inserted into the bore14 of the shank 4, such that the plugging portion of the breakstemcontacts the tail end 12 of the fastener shank 4. The elongate shank 88is then pulled relative to the fastener 2 thereby causing the breakstemplugging portion 82 to enter the fastener shank 4, simultaneouslycausing the fastener shank 4 to expand and the plugging portion 82 tocollapse inwardly to a small degree due to the hollow core 84. Theelongate shank 88 is pulled until the breakstem 80 fails at thebreakneck point 86.

The slight inward collapse of the plugging portion 82 allows for avariation in workpiece aperture dimension, i.e. the plugging portion 82is more compliant than the solid core of the breakstem 70 of FIGS. 13and 14. This method of installation therefore provides an enhancedtolerance to differences in workpiece aperture diameter. The breakstem80 could include a locking “skirt” feature (not shown) which is createdby the parallel portion 85 on installation of the fastener, whereinmaterial of the parallel portion 85 is displaced radially outwards intothe countersink in the fastener head 6, i.e. into the area marked at 87in FIG. 16. The locking skirt thereby provides a mechanical lock of theinstalled breakstem 80 in the fastener shank 4 to resist recoil of theplugging portion 82 on failure of the breakneck point 86 and againstremoval of the plugging portion 86 from the fastener shank 4 afterinstallation.

The method illustrated in FIG. 17 is a “push broach” method, wherein asolid pin 90, having a diameter larger than that of the fastener bore14, is driven into the fastener 2 via the head 6 to expand the fastenershank 4. This pin 90 could remain in the fastener shank 4 permanently orcould be a removable part of the placing tooling, in which case asupport sleeve (not shown) might be needed around the pin 90 to bear onthe fastener head 6 to allow removal of the pin 90 from the fastenershank 4.

In the alternative fastener 2′ as illustrated in FIGS. 18 and 19, thecountersink 26′ comprises a curved tapered annular wall, and the taperedneck portion 28 is not present. This embodiment radius reduces thelikelihood of debris generation on installation of the fastener. Thebroach load caused when the mandrel head passes through the fastenerhead 6 is reduce, whilst maintaining sufficient tensile strength. Duringinstallation, some material of the fastener shank 4 is displacedbackwards towards the tool, such that the counterbore is no longervisible in the placed fastener.

A further alternative fastener (not shown in the figures) could comprisea countersink having a curved tapered annular, and a neck portion.

In a further alternative fastener 2″, as illustrated in FIGS. 20 to 22,an end section 60 having a straight annular wall which is parallel tothe longitudinal axis of the fastener bore 14, is provided beyond thetapered point 10 at the tail end of the fastener shank 4. In thisembodiment, the countersink 26″ comprises a flat annular wall which isparallel to the longitudinal axis of the fastener bore 14, i.e. the wallof the countersink 26″ is not tapered. A tapered neck portion 28 is alsoprovided between the countersink 26″ and the fastener bore 14. Thediameter of the countersink 26″ is similar to the diameter of the endsection 60 to allow the fasteners to be arranged “head-to-tail” in astack 50′ of the fasteners, as illustrated in FIGS. 23 and 24.

What is claimed is:
 1. A fastener, for securing a workpiece comprising aplurality of workpiece members, said fastener comprising a shank with ahead end and a tail end remote from the head end, and a radiallyenlarged head at the head end of the shank, and a fastener boreextending throughout the fastener, wherein an external wall of the shankcomprises a voided section comprising a plurality of voids separated bya plurality of splines each of which culminates in a crest, the voidsand splines being elongated in an axial direction with respect to alongitudinal axis of the fastener bore; wherein the splines are of anapproximate trapezoidal cross-section, wherein the voids have across-sectional area which is constant along the length of the voidedsection of the fastener shank, and wherein the shank includes anunvoided solid tail ring between the voided section and the tail end ofthe shank, the unvoided solid tail ring having an external diameterapproximately equal to the external diameter of the shank in the voidedsection.
 2. A fastener as claimed in claim 1 wherein the axial voids areequidistant from one another.
 3. A fastener as claimed in claim 1wherein the axial voids extend from immediately underneath the fastenerhead.
 4. A fastener as claimed in claim 1 wherein the fastener headcomprises a countersink provided in a top surface of the head remotefrom the fastener shank, the countersink having an average diameterwhich is greater than the diameter of the fastener bore.
 5. A fasteneras claimed in claim 4 wherein, the countersink comprises a flat annulartapered wall.
 6. A fastener as claimed in claim 4 wherein thecountersink comprises a curved tapered annular wall.
 7. A fastener asclaimed in claim 4 wherein the countersink comprises a flat annular wallwhich is parallel to the longitudinal axis of the fastener bore.
 8. Afastener as claimed in claim 4 wherein a tapered neck portion isprovided between the countersink and the fastener bore.
 9. A fastener asclaimed in claim 1 further comprising a tapered point at the tail end ofthe fastener shank.
 10. A fastener as claimed in claim 9 furthercomprising an end section at the tail end of the fastener beyond thetapered point, the end section having a straight annular wall which isparallel to a longitudinal axis of the fastener.
 11. A fastener asclaimed in claim 10, wherein the diameter of the end section is lessthan the diameter of a countersink provided in a top surface of the headremote from the fastener shank, the countersink having an averagediameter which is greater than the diameter of the fastener bore.
 12. Afastener as claimed in claim 1 further comprising a solid tail ring atthe tail end of the fastener shank, between the voided section and thetapered point.
 13. A method of installing a fastener according to claim1, thereby to secure a workpiece comprising a first a pertured workpiecemember and a second apertured workpiece member, the method comprisingthe steps of; a) placing a mandrel having a radially enlarged headthrough a fastener bore such that the radially enlarged head of themandrel is adjacent a tail end of a fastener shank; b) inserting thefastener and mandrel into the apertures in the first and secondworkpiece members such that the fastener head contacts the firstworkpiece member; c) supporting the fastener at a head end whilstdrawing the mandrel entirely through the fastener bore, therebyexpanding the fastener shank into the apertures of the workpiecemembers, and causing the crests of the splines to be deformed againstthe apertures, and causing the tail end of the fastener shank toradially enlarge adjacent to the second workpiece member.
 14. A methodas claimed in claim 13 wherein step c) is undertaken by a toolcomprising a conical concave end face which bears on an upper surface ofthe fastener head.
 15. A method of installing a fastener according toclaim 1, thereby to secure a workpiece comprising a first aperturedworkpiece member and a second apertured workpiece member, the methodcomprising the steps of; a) inserting a stem comprising a radiallyenlarged head, a breakneck point, and a plugging portion, into thefastener through the fastener bore such that the radially enlarged headof the stem is adjacent the tail end of the fastener shank; b) insertingthe fastener and the stem into the apertures in the first and secondworkpiece members such that the fastener head contacts the firstworkpiece member; c) supporting the fastener at the head end whilstpulling the stem with respect to the fastener head, thereby causing theplugging portion to radially expand the fastener shank into theapertures of the workpiece members and deform the crests of the splinesagainst the apertures, and also thereby causing the radially enlargedhead of the stem to contract, and then compress, the tail end of thefastener shank, thereby causing the tail end of the fastener shank toradially enlarge adjacent to the second workpiece member; wherein thestem is pulled with respect to the fastener head through the fastenerbore until the stem fractures at the breakneck point.
 16. A method ofinstalling a fastener according to claim 15, wherein the stem furthercomprises a parallel portion between the breakneck point and theplugging portion, and wherein, during step c), a locking skirt iscreated by the parallel portion, thereby providing a mechanical lock ofthe installed stein in the fastener shank.
 17. A method of installing afastener according to claim 1, thereby to secure a workpiece comprisinga first apertured workpiece member and a second apertured workpiecemember, the method comprising the steps of; a) inserting the fastenerinto the apertures in the first and second workpiece members such thatthe fastener head contacts the first workpiece member; b) inserting abreakstem comprising a plugging portion with a hollow core, an elongateshank, and a breakneck point between the plugging portion and theelongate shank, into the bore of the fastener, such that the pluggingportion of the breakstem contacts the tail end of the fastener shank; c)pulling the elongate shank of the breakstem pulled relative to thefastener, thereby causing the breakstem plugging portion to enter thefastener shank, causing the fastener shank to expand into the workpieceapertures, and causing the crests of the splines to be deformed againstthe apertures, and simultaneously causing the plugging portion tocollapse inwardly; wherein the elongate shank is pulled relative to thefastener until the breakstem fails at the breakneck point.
 18. A methodof installing a fastener according to claim 1, thereby to secureworkpiece comprising a first apertured workpiece member and a secondapertured workpiece member, the method comprising the steps of; a)inserting the fastener into the apertures in the first and secondworkpiece members such that the fastener head contacts the firstworkpiece member; b) driving a solid pin, having a maximum diameterlarger than that of the fastener bore, into the bore of the fastener viathe head thereby to radially enlarge the fastener shank into mechanicalengagement with the workpiece apertures, and causing the crests of thesplines to be deformed against the apertures.
 19. A method of installinga fastener according to claim 18 further comprising a subsequent step ofbearing on the fastener head by a support sleeve, and removing the pinFrom the fastener bore.
 20. Fastening apparatus, for securing aworkpiece comprising a plurality of workpiece members, said apparatuscomprising a fastener and a mandrel; said fastener comprising a shankwith a head end and a tail end remote from the head end, and a radiallyenlarged head at the head end of the shank, and a bore extendingthroughout the fastener; wherein an external wan of the shank comprisesa voided section comprising a plurality of voids separated by aplurality of splines each of which culminates in a crest, the voids andsplines being elongated in an axial direction with respect to alongitudinal axis of the fastener bore; wherein the splines are of anapproximate trapezoidal cross-section; wherein the voids have across-sectional area which is constant along the length of the voidedsection of the fastener shank; and wherein the shank includes anunvoided solid tail ring between the voided section and the tail end ofthe shank, the unvoided solid tail ring having an external diameterapproximately equal to the external diameter of the shank in the voidedsection.
 21. Fastening apparatus as claimed in claim 20 wherein saidmandrel comprises a radially enlarged head of a maximum diameter whichis greater than a diameter of the fastener bore, and less than a maximumexternal diameter of the fastener shank.